1. Field of the Invention
The present invention relates to information processing equipment for inhibiting electromagnetic interference (hereinafter referred to as xe2x80x9cEMIxe2x80x9d) and, in particular, it relates to information processing equipment, such as a personal computer, for inhibiting electromagnetic noise from printed circuit boards.
2. Description of the Related Art
Conventionally, in information processing equipment such as desktop or notebook personal computers, it is required to inhibit electromagnetic noise. However, in such information processing equipment, as the operating frequency increases, it becomes more and more difficult to prevent the electromagnetic noise or EMI from leaving printed circuit boards.
On the other hand, for the purpose of preventing the EMI, along with legal regulations in each country, international standards such as CISPR (Comite International Special des Pertubations Radioelectrique, or The International Special Committee on Radio Interference in the IEC (International Electrotechnical Commission)) are established and, in Japan, a domestic standard is specified by VCCI (Voluntary Control Council for Interference by Information Technology Equipment). In order to satisfy such standards, it is important to reduce undesired electromagnetic radiation.
Conventionally, a printed circuit board used for a main board of a personal computer is mounted on a plurality of bosses provided on the bottom surface of a case, and is fastened by screws onto the bosses, wherein a signal ground pattern (hereinafter referred to as the xe2x80x9cSG patternxe2x80x9d) of the printed circuit board is connected to a frame ground on the bottom surface of the case via the bosses so that impedance is lowered to prevent the EMI.
In order to reduce the electromagnetic radiation due to resonance by connecting the SG pattern of the printed circuit board to the frame ground as described above, it is necessary to establish an appropriate distance between connection points according to the operating frequency in use. At present, as the operating frequency increases as CPUs become faster, for the prevention of EMI, it is necessary to connect the SG pattern of the printed circuit board to the frame ground of the case at multiple points and at minimum spacing ({fraction (1/10)} or less of a clock frequency wavelength xcex). As an example, as the wavelength is 30 cm at a clock frequency of 1 GHz, the spacing between the connection points must be as small as 3 cm or less.
However, when the spacing is reduced by increasing the number of the conventional bosses for mounting the printed circuit board, there is a problem in that productivity is reduced as the area cost is increased and the mounting efficiency is reduced because more screws must be used.
In order to solve such problem, the following technique is known.
Unexamined Patent Publication (Kokai) No. 10-255912 (published on Sep. 25, 1998) discloses a technique for preventing undesired electromagnetic radiation by forming a ground pattern inside a printed circuit board and bringing a contact plate spring member in a connector housing into contact with said ground pattern. Thus, it discloses that a line or a surface contact, instead of a point contact, between the SG pattern and the frame ground is effective for preventing electromagnetic radiation.
However, in this technique, the SG pattern is grounded to the frame ground from one side of the connector via the housing.
Further, Unexamined Patent Publication (Kokai) No. 8-228088 (published on Sep. 3, 1996) discloses a technique for establishing electrical connection between a printed circuit board and a shield case by holding the printed circuit board between Q-shaped protruding guides at three sides of the shield case.
However, this technique necessitates a special process, for the shield case itself, for connection to the printed circuit board, and only three sides of the shield case are shielded.
Still further, examples of prior art include the following.
Unexamined Patent Publication (Kokai) No. 11-220263 discloses a printed circuit board wherein a plurality of ground layers are provided and a power supply layer and a signal layer are sandwiched by the plurality of ground layers.
Unexamined Patent Publication (Kokai) No. 9-64581 discloses a single-layer printed circuit board wherein a grounded circumferencial circuit is provided along the circumference of the board, as well as a multi-layer printed circuit board wherein there is a through-hole that communicates with a grounded circumferencial circuit that is provided at the circumference of the board surface.
Unexamined Patent Publication (Kokai) No. 8-162853 discloses a technique for connecting ground patterns, formed as a plurality of inside layers of a multi-layer printed circuit board, to each other by through-holes and metalized walls.
In view of the above problem, it is an object of the present invention to provide information processing equipment using a versatile grounding plate that can shield four sides of a printed circuit board securely, and with a simpler configuration, for inhibiting or preventing EMI due to electromagnetic noise in information processing equipment.
In order to achieve the above object, the present invention provides information processing equipment, comprising a printed circuit board mounted in a case, wherein a signal ground pattern is formed at the circumference of the printed circuit board, a connector mounting plate is electrically fixed to one side of the printed circuit board and connected to the signal ground pattern, grounding plates that comprise elastic contacts having a connecting spacing that is smaller than a value determined by a signal wavelength are disposed at the other three sides of the printed circuit board, and the signal ground pattern of the printed circuit board is grounded to a frame pattern of the case.
Further, there is also provided information processing equipment, comprising a printed circuit board mounted in a case, wherein a signal ground pattern is formed at the circumference of the printed circuit board, signal ground vias are provided in the signal ground pattern, a connector mounting plate is fixed to one side of the printed circuit board and electrically connected to the signal ground pattern, and grounding plates that comprise comb-like contacts having a connecting spacing that is smaller than a value determined by a signal wavelength are disposed at the other three side of the printed circuit board so that the signal ground pattern is grounded to a frame pattern of a case by inserting and electrically connecting the comb-like contacts to the signal ground vias.